\section{Introduction} \label{introduction}

%\emph{
%First paragraph: Role of Overspecification in reference, brief overview of the previous work \\
%Second: Possible explanations for Overspecification  \\
%Third: Our hypothesis for overspecification, conceptual pacts, link with REs and lexical acquisition. SV: Is it clearer now? \\
%Fourth: The experiment, how it works, proof of second hypothesis with results of the experiment.  \\
%Fifth: Alignment \\
%Sixth: Dialogue systems? TBD. LB: Let me work on this when you finish \\
%Seventh: Plan of the paper\\}

The study of the generation and interpretation of \emph{referring expressions} (REs) has been an active area of research for many years, due to its importance for communication~\cite{Clark_Wilkes-Gibbs_1986}. Indeed, in the past few years, there has been an increased focus on this area~\cite{Krahmer-Van-Deemter-2011,Deemter_2009,Kelleher_2007}. In particular, the role of \emph{overspecification} in reference has received much attention~\cite{Arts_Maes_Noordman_Jansen_2008,Koolen_Gatt_Goudbeek_Krahmer_2011}. Several studies~\cite{Pechmann_1989,Nadig_Sedivy_2002,Engelhardt_Bailey_Ferreira_2006} have shown that, although it makes interpretation more costly, redundant information is frequently used in the referring expressions produced by speakers. 
%In practice, the extent and circumstances of overspecification are hard to define, while numerous attempts have been made to describe this phenomenon \cite{Arts_Maes_Noordman_Jansen_2011,Sluiskrahmer2005}. 

In the relevant literature, there are two main competing explanations that have been proposed for the overspecification phenomenon. One explanation holds that (1) the cognitive effort of producing non-redundant referring expressions is too high~\cite{Pechmann_1989}. Pechmann's \emph{principle of least effort} suggests that it is not cognitively efficient for speakers to wait for the evaluation of the context that is necessary to produce a minimal specification. According to this view, overspecification is a result of a cognitive limitation and impairs the comprehension of the REs~\cite{Engelhardt_Bar_11}. 
The other explanation claims that (2) overspecification is actually a useful part of communication because it gives the listener more chances to align with the speaker, compensates for perceptual difficulties, and makes future communication more effective~\cite{Nadig_Sedivy_2002}. Explanation 2 is coherent with psycholinguistic findings that show that listeners do not rate overspecified REs any worse than minimal ones~\cite{Engelhardt_Bailey_Ferreira_2006}. The usefulness of overspecification has not been empirically shown yet.

In this paper, we aim to support explanation (2) by empirically evaluating the effect of overspecification on lexical acquisition in second language (L2) learning. Our hypothesis is that overspecification helps establish \emph{conceptual pacts}~\cite{Brennan_Clark_1996} between the speaker and the listener, which in turn facilitates lexical acquisition. By giving more information than is strictly needed to identify the object, the speaker aims to optimize the chances of alignment with his listener. In the case of a reference, they maximize the chance of the referent being found. Our  hypothesis for this experiment is that giving overspecified REs during practice exercises will aid lexical acquisition.

To test our hypothesis, we created an instruction-giving system that produces minimal and overspecified REs of objects located in the context of a 3D virtual world. We chose a virtual environment because it is more immersive than flashcards or images, and closer to real-world object perception. Furthermore, the interpreter's visibility area and movements towards the referents can be registered with precision, giving evidence of their interpretation process. Our study was done on subjects learning new vocabulary in Russian.  We divided the subjects into two groups: one received minimal REs during practice exercises, and the other received overspecified REs. We aimed to see if, by using overspecified REs during the exercise period of lexical acquisition, the effectiveness of vocabulary learning is increased, compared to minimal REs. If this is indeed the case, we will have offered empirical evidence to support the explanation (2) described above.

Whether explanation (1) or (2) is appropriate, has implications for the design of dialogue systems. If overspecification is a result of the speaker's limited cognitive resources, and causes unjustified extra effort in the listener, we could argue that dialogue systems should only produce minimal REs. Current algorithms for RE generation take this approach and generate minimal descriptions~\cite{Krahmer-Van-Deemter-2011}. On the contrary, if overspecification is useful for building conceptual pacts that could be used later on in communication, dialogue system developers should consider including algorithms that produce overspecified references.  

The rest of the paper proceeds as follows. In Section~\ref{theoretical-background}, we review previous work in the area of RE generation and interpretation and studies related to overspecification. In Section~\ref{experimental-setup}, we describe the various practical aspects of our experiment: the participants, the virtual world used, and the procedure. We present both the objective and subjective results of the experiment in Section~\ref{results}, specifying the metrics we studied and their pertinence to evaluating our hypothesis. The discussion of the results follows in Section~\ref{discussion}, where we compare the results of the two groups of subjects studied. Finally, Section~\ref{conclusion} concludes and presents future work. 

